Concur, this is the idea that finally made me buy into WSGI a while back: lots of modular components talking (like) HTTP up and down, perhaps minimally functional on their own or just web-like intermediaries.
In a (yet another) small side project that will probably never get beyond the barely functional prototype let alone see the outside world, I'm experimenting with implementing a REST-like interface to a relational database as a WSGI app, outputting simplistic XHTML representations of the data (rows/records), and then I have a minimal intermediary that queries this app to combine/restrict datasets or follow link references for parent-detail display. If I go much further in splitting things out (separate apps for each relational algebra operator?) I'll probably end up with a great example of what NOT to do with WSGI, but it's liberating in some ways.
Definitely some mental mismatch at this point in doing all subprocessing against the HTML output of upstream apps, but the less extreme cases for compositing miniapps using WSGI are completely right in my mind now.
Clark Evans, who has been a Paste contributor, was using it for an application exactly like you describe -- SQL over HTTP. It's not open source, but I thought the was trying to get it open sourced... but I don't know if that worked out ultimately.# Ian Bicking